<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>2-2 border</title>
  <style>
    .wrapper {
      display: flex;
    }

    .box1 {
      width: 0;
      height: 0;
      border-left: 50px solid transparent;
      border-right: 50px solid transparent;
      border-bottom: 100px solid #e55039;
    }

    .box2 {
      width: 0;
      height: 0;
      border-top: 50px solid transparent;
      border-bottom: 50px solid transparent;
      border-left: 100px solid #e55039;
    }

    .box3 {
      width: 0;
      height: 0;
      border-left: 50px solid transparent;
      border-right: 50px solid transparent;
      border-top: 100px solid #e55039;
    }

    .box4 {
      width: 0;
      height: 0;
      border-top: 50px solid transparent;
      border-bottom: 50px solid transparent;
      border-right: 100px solid #e55039;
    }
  </style>
</head>

<body>
  <div class="wrapper">
    <div class='box1'></div>
    <div class='box2'></div>
    <div class='box3'></div>
    <div class='box4'></div>
  </div>
</body>

</html>